1. Field of Invention
The invention relates to a light-emitting diode and the manufacturing method for it. More particularly, it relates to a light-emitting diode and the corresponding fabrication method with a lower manufacturing difficulty and cost.
2. Related Art
The light-emitting diode (LED) uses the intrinsic property of semiconductors to emit light, whose principle is different from that of a normal lamp. Therefore, the LED is called a clod light source. Moreover, the LED has the advantages of being durable, light, energy-efficient, and not having mercury. Therefore, the illumination industry likes LED's very much.
Generally speaking, the LED is usually formed from epitaxy of the III-V family mixed crystal compounds such as GaP and GaN. Since the index of refraction of the LED's is greater than the exterior and the conventional LED's are mainly cubic in shape, light beams generated by the LED with an incident angle larger than the threshold angle will be totally reflected at the diode/air interface back into the diode. The four interfaces of the cubically symmetric diode crystal are parallel to one another, causing the above-mentioned light beams to totally reflect within the LED. As a result, the light-emitting efficiency of the LED is much lower than its internal quantum efficiency. A solution to this deficiency is to change the crystal shape of the LED.
In accord with the current semiconductor machining technology, the first example that has successfully used this method is disclosed in the U.S. Pat. No. 6,229,160, a truncated inverted pyramid (TIP) LED. According to that patent, the side surfaces of the AlGaInP/GaP LED crystal are made into a pyramid shape so that the four interfaces are not parallel to one another. This configuration can effectively guide the beams out, increasing the light-emitting efficiency by a factor of two. However, the TIP LED is formed by direct mechanical machining. This method can only apply to AlGaInP/GaP red diodes. It utilizes the feature that a four-element material can be conveniently machined to form the TIP LED. The GaN LED's, on the other hand, are mostly epitaxy on sapphires. As the sapphires are very hard, mechanical machining is almost impossible. There has not been any breakthrough in this direction.
The U.S. Cree, Inc. is successful in making TIP LED's using GaN. They use the property that machining for SiC is much easier than that for sapphires to polish the SiC substrate. Using this method, the GaN LED can be made into a TIP shape. However, the crystal lattices of GaN and SiC, and the SiC substrate will absorb ultraviolet (UV) light, affecting the light-emitting efficiency of the UV GaN LED. Nevertheless, the white-light LED based upon the UV GaN LED is believed to be an ideal illuminating material in the next generation.
Although directly using the GaN substrate to make the InGaN/GaN LED can be made into the TIP shape using mechanical machining, the slant surfaces formed by mechanical machining will contain a layer with residual stress after the InGaN/GaN epitaxy is grown on the GaN substrate. Such a layer absorbs light and is not easy to be removed. Therefore, it is harmful to the LED. Furthermore, the GaN substrate has a lower yield and a higher cost. Its market price is much higher than the SiC substrate and the sapphire substrate. Therefore, it is hard to be commercialized.